In response to popular demand for easy-to-enjoy music, a variety of technologies have been developed in recent years for performing compression encoding for audio signals such as voice and musical sounds at low bit rates and performing decompression decoding when reproducing these signals. A representative example of such technologies is the MPEG AAC system (to be abbreviated as “AAC” hereinafter) (Refer to: M. Bosi, et al.: “IS 13818-7 (MPEG-2 Advanced Audio Coding, AAC),” April, 1997)
FIG. 1 is a diagram showing a frequency band to be encoded in the AAC system.
However, since an increased compression rate results in a lower upper limit frequency of the reproduction band, no high frequencies can be reproduced. For, as a compression rate increases, a sufficient number of bits for encoding the high frequency band cannot be allocated, making the upper limit of the reproduction band lower.
Against this backdrop, recent years have witnessed technological development of as well as standardization for pseudo wide band as part of the standardization effort of MPEG4 Ver. 3, with the view to cover such lack of signals at high frequencies.
As shown in FIG. 2, the above-mentioned technology is intended, for example, to cover the lack of signals at high frequencies using band information of the narrow band, that is, information at low frequencies to predict high frequency information. The use of such technology with which pseudo wide band is created makes it possible to listen to high-quality music and watch news on such a battery-operated device as a mobile phone.
However, the constant provision of high-quality sounds ends up meaningless in many cases. To put it another way, when listening to news, for example, there are fewer user requests for reproducing sounds for which pseudo wide band is created, meaning that it is impractical for a decoding device to perform pseudo wide band processing. Furthermore, it results in a waste of battery power of a mobile phone and other devices embedded with a decoding device, which performs pseudo wide band processing even when there is no user request for this processing.
The present invention is intended to solve such problems whose first object is to provide a decoding device capable of eliminating the redundancy of listening to high-quality sounds all the time even when it is not desired.
The second object of the present invention is to provide a decoding device that allows the use of a smaller amount of battery energy when a digital signal (to be referred to also as a “PCM signal” hereinafter) of sounds in the narrow band is reproduced.
The third object of the present invention is to provide an encoding device and a system facilitating the achievement of the above first and the second objects.